The present invention relates generally to spacecraft communication systems and methods, and more particularly, to array fed multiple beam antenna systems and methods for use in spacecraft communication systems.
The assignee of the present invention manufactures and deploys communication satellites. In order to provide desired coverage of a particular area on the Earth, and maximize re-use of the allocated frequency spectrum, it is necessary to use a multiple beam antenna system.
Conventional multiple beam antenna systems that provide contiguous coverage of a desired region, typically localize antenna beams on a two dimensional triangular or rectangular lattice. Conventional reflector or lens multiple beam antenna systems generally require the use of three or four apertures to efficiently achieve the desired coverage. Furthermore, the bandwidth for each beam produced by conventional multiple beam antennas and useable in a frequency re-use plan is generally less that would be desired.
Previous designs for multiple beam antennas use a single horn radiator for each feed in the antenna. The single feed radiator design used in previous multiple beam antenna designs was a compromise that minimized the worst case scan beam degradation. This caused either poor performance for the beams close to focus, or poor performance for the scan beams.
The previous designs thus suffered from the effect of scan aberration that could not be corrected by modifying the field distribution in the focal plane of the antenna. These designs are well known and published extensively throughout the literature relating to antenna design. The present invention avoids this compromise by allowing different feed radiator characteristics to be used for different beam positions.
It would be desirable to have a multiple beam antenna system and communication methods for use with a communications satellite. It would also be desirable to have a multiple beam antenna system for use with a communications satellite that allows different feed radiator characteristics to be used for different beam positions. It is therefore an objective of the present invention to provide for array fed multiple beam antenna systems and methods for use in satellite communication systems.
To accomplish the above and other objectives, the present invention provides for array fed multiple beam antenna systems and methods that improve upon conventional multi-beam antenna systems and beam generation methods. An exemplary system is employed in a communications system disposed on a spacecraft and comprises a reflector and an array feed, such as a waveguide slot array or an array of small horns. The array feed is relatively small compared to the reflector. The array feed has a plurality of feeds that illuminate the reflector. Each of the feeds includes a plurality of radiators and a power division network that excites each radiator of the respective feeds.
The radiators of each feed cluster may be disposed in a square or rectangular pattern. The radiators are disposed in a focal plane of the reflector. Each individual array feed is used for each respective beam position. Excitation coefficients used for each array feed, which correspond to different secondary beams from the reflector, may be different.
The excitation coefficients used for each array feed may be fixed prior to launching the spacecraft into orbit. Alternatively, the excitation coefficients may be variable to tune interbeam isolation. The excitation coefficients may be varied by adjusting the amplitude and phase coefficients while the spacecraft is in orbit using variable phase shifters and variable power dividers.
The antenna system is capable of very wide scan angle operation. The phase aberration normally associated with scanning is corrected by adjusting the excitation coefficients of each array feed. An antenna configuration that would normally be suitable for narrow angle scanning, such as regional coverage of a single country, for example, can therefore be used to provide multiple spot beam coverage over the surface of the Earth viewed from a synchronous orbit spacecraft.
In implementing an exemplary method, a spacecraft is launched into orbit that carries a communication system having a multiple beam antenna system. The multiple beam antenna system includes a reflector and the array feed having a plurality of radiators coupled to the communication system by way of a power division network. For optimum performance with the antenna system is operated at two frequency bands (such as a transmit band and a receive band), a frequency selective surface (FSS) may be used to allow individual optimization of two different feed arrays, for the two different operating bands.
Use of the frequency selective surface provides an efficient interface between the transmit feed arrays and power amplifiers that drive them. Use of the frequency selective surface allows the transmit feed arrays to be located relatively close to the power amplifiers. Therefore, relatively short waveguide transmission lines are used between the power amplifiers and the transmit feed arrays. More power is delivered to the transmit feed arrays and less loss is experienced by the communications system.
During transmission, RF energy is coupled from the communication system by way of the power division network to the radiators of the respective feeds to excite each of radiators. Energy radiated by the radiators is reflected by the reflector to produce multiple spot beams. The spot beams are scanned across a field of regard by controlling the position of each array feed in the focal plane and using the appropriate the amplitude and phase distribution associated with a particular spot beam (array feed).
Controlling the amplitude and phase distributions produced by the radiators allows different focal plane distributions to be realized for different scan positions to optimize the beamshapes generated by the multiple beam antenna system over a very wide coverage region. As was stated above, the amplitude and phase distribution associated with the respective array feed is typically fixed, although variable distributions may be implemented.
During reception, multiple spot beams are reflected by the reflector to the radiators of respective elements of the feeds. The RF energy contained in the multiple spot beams is coupled by way of the power combining network to the communication system.
Thus, the present invention uses a small array radiator for each individual feed in a multiple beam antenna system. One advantage of using a small array as the elemental radiator in a multiple beam antenna is that it provides for control of the amplitude and phase distribution within the focal plane cell that corresponds to a radiated beam from the multiple beam antenna. The use of the small array allows different distributions to be realized for different scan positions which optimizes the beamshapes generated by the multiple beam antenna over a very wide coverage region.